1. Field of the Invention
The present invention relates to a manufacturing method of an opto-electric hybrid board including an optical waveguide and an electric circuit mounted with an optical element.
2. Description of the Related Art
Conventionally, an opto-electric hybrid board is produced by separately preparing an electric circuit board and an optical waveguide and then bonding the electric circuit board and the optical waveguide to each other with an adhesive. As shown in FIG. 6, the opto-electric hybrid board includes, for example, an electric circuit board 81 including a multi-level electric circuit 83, and an optical waveguide 80 including an under-cladding layer 86, a core 87 and an over-cladding layer 88 and bonded onto the electric circuit board 81 with an adhesive 82. A light emitting element 11 and a light receiving element 12 are mounted on mount pads provided on the electric circuit board 81 (as parts of the electric circuit 83) by a flip-chip mounting method (see, for example, Japanese Patent Application Laid-Open No. 2000-199827). In the opto-electric hybrid board shown in FIG. 6, the light emitting element 11 and the light receiving element 12 are respectively positioned at opposite end portions of the optical waveguide 80 on the front side. The opposite end portions of the optical waveguide 80 each have a tilt surface tilted at 45 degrees with respect to the optical axis. A portion of the core 87 present in the tilt surface serves as a light path deflection mirror 87a. In FIG. 6, a reference character 11a denotes electrodes (bumps) of the light emitting element 11, and a reference character 12a denotes electrodes (bumps) of the light receiving element 12.
In the opto-electric hybrid board, a light beam L is transmitted in the following manner. First, the light beam L is emitted downward from the light emitting element 11. The light beam L passes through the over-cladding layer 88 at one of the opposite end portions (at a left end portion in FIG. 6) of the optical waveguide 80 to be inputted into one of opposite ends of the core 87. Then, the light beam L is reflected (deflected 90 degrees) on the light path deflection mirror 87a at the one end of the core 87, and travels axially of the core 87. The light beam L is transmitted through the core 87 to reach the other end (a right end in FIG. 6) of the core 87. Subsequently, the light beam L is reflected upward (deflected 90 degrees) on the light path deflection mirror 87a at the other end, and passes through the over-cladding layer 88 to be outputted and received by the light receiving element 12.
In practice, when the electric circuit board 81 and the optical waveguide 80 are bonded to each other with the adhesive 82 in the production of the opto-electric hybrid board of Japanese Patent Application Laid-Open No. 2000-199827, the electrical circuit board 81 and the optical waveguide 80 are slightly offset from each other due to the fluidity of the adhesive 82 by a pressure applied thereto for the bonding. Therefore, even if the light emitting element 11 and the light receiving element 12 are accurately mounted on the electric circuit board 81 by detecting the mount pads of the electric circuit board 81, the light emitting element 11 and the light receiving element 12 are positioned with respect to the light path deflection mirrors 87a at the opposite ends of the core 87 of the optical waveguide 80 with a reduced alignment (positioning) accuracy because of the offset of the electric circuit board 81. In this state, the light beam L is transmitted with the optical axes of the light emitting element 11 and the light receiving element 12 being misaligned with the light path deflection mirrors 87a at the opposite ends of the core 87, so that a light coupling loss is increased.
In view of the foregoing, it is an object of the present invention to provide an opto-electric hybrid board manufacturing method which improves the alignment accuracy of an optical element with respect to a core of an optical waveguide.
To achieve the object described above, the inventive opto-electric hybrid board manufacturing method includes the steps of: preparing an electric circuit board; fabricating an optical waveguide by forming an optical waveguide formation photosensitive resin layer on a surface opposite from a circuit formation surface of the electric circuit board and patterning a core formation region of the photosensitive resin layer by a photolithography method to form a core having a predetermined pattern; mounting an optical element on a part of the circuit formation surface of the electric circuit board in association with an end of the optical waveguide; forming a reflection portion on an end portion of the core located at the end of the optical waveguide, the reflection portion serving to reflect a light beam to permit light transmission between the core and the optical element; and forming a passage in the electric circuit board for the light transmission between the core and the optical element; wherein a photosensitive resin layer including an alignment mark formation region in addition to the core formation region is used as the photosensitive resin layer, and the alignment mark formation region is patterned by the photolithography method to form an alignment mark having a predetermined pattern simultaneously with the formation of the core in the optical waveguide fabricating step; wherein the optical element is mounted at a predetermined position with reference to the alignment mark in the optical element mounting step.
In the inventive opto-electric hybrid board manufacturing method, the photosensitive resin layer having the core formation region and the alignment mark formation region is formed on the surface opposite from the circuit formation surface of the electric circuit board, and the photosensitive resin layer is patterned once by the photolithography method to simultaneously form the core and the optical element positioning alignment mark. Thus, the optical waveguide is fabricated from the photosensitive resin layer. Therefore, there is no need to use an adhesive for bonding the optical waveguide to the electric circuit board, thereby eliminating the drawback associated with the offset which may otherwise occur when the adhesive is used. At the same time, the core can be positioned in predetermined positional relation with respect to the alignment mark. Since the mounting of the optical element is achieved with reference to the alignment mark in the present invention, it is possible to mount the optical element in proper positional relation with respect to the core of the optical waveguide. As a result, the light coupling loss occurring between the optical element and the light reflection portion at the end of the core can be minimized on the opto-electric hybrid board thus produced.